This invention relates to the demetallization of hydrocarbon charge stocks. More particularly, it relates to the removal of metallic contaminants, especially those of nickel and vanadium, from residual oils.
Various petroleum feedstocks, such as crude petroleum oils, heavy vacuum gas oils, shale oils, oils from bituminous sands, topped crudes and atmospheric or vacuum residual fractions contain varying amounts of non-metallic and metallic impurities. The non-metallic impurities usually include nitrogen, sulfur and oxygen; the metallic impurities usually include nickel, vanadium, iron, sodium, copper, zinc and arsenic. In addition, it is known that most of the metallic contaminants are present as inorganic sulfides, oxides and water soluble constituents, while the remainder is usually in the form of relatively thermally stable organometallic complexes such as metal porphyrins and derivatives thereof. It is generally conceded by the art that for purposes of demetallization the removal of the organometallics is more difficult than that for the inorganics.
The presence of these contaminants in petroleum feedstocks presents assorted difficulties in the refining process. In most petroleum refineries today, however, the problems associated with the inorganic metallics and non-metallic impurities have largely been resolved. Catalytic hydrotreating or hydrofining operations effectively remove sulfur, nitrogen and oxygen from petroleum stocks by hydrodecomposition. Inorganic metallics, on the other hand, are generally removed at least in part, by filtration, water washing or electric desalting operations performed prior to hydrofining. But removal of the organometallics, especially those of vanadium and nickel, is especially troublesome. These components are readily adsorbed by conventional hydrofining catalysts and other catalysts used in cracking and reforming operations, resulting in rapid deactivation thereof.
A general method by which these organometallics can be at least partially removed prior to the above catalytic contacting processes is to pass the desalted feedstock (especially those with metals content in excess of 150 ppm) over adsorptive and/or catalytic materials. Many such materials are known in the art. For example, spent cobalt-molybdate hydrotreating catalysts have been used for this purpose but generally they are too badly coked to provide effective service. Bauxite has also been used for this purpose as reported in U.S. Pat. No. 2,687,985. Manganese nodules removed from the sea floor have been reported in a series of patents (see U.S. Pat. Nos. 3,772,185, 3,716,479, 3,766,054 and 3,813,331) as being effective for demetallizing petroleum feedstocks. However, the role of the manganese in these nodules is yet to be determined, inasmuch as the first of the aforementioned references indicates that the nodules performed better after some or all of the manganese had been leached out. A composite of 1% manganese on a large pore bauxite (more than 60% of pore volume in pores of greater than 200 A diameter) has also been used for demetallization (Demetallization of Heavy Residual Oils by Rovesti & Wolk, U.S. Environmental Protection Agency, Washington, D. C. 20460, EPA-650/2-73-041 published by U.S. Department of Commerce).
Regardless of the catalytic material used, it is most desirable that it meet the twin criteria of inexpensiveness and ability to demetallize sufficiently to extend the useful life of the hydrotreating or cracking catalyst, preferably to the extent that they become deactivated due to other factors, e.g., aging, coking, etc. This goal may be accomplished even if the demetallization is only between 25 and 50% complete because the life of the hydrotreating catalyst, for example, increases more than proportionally with the fraction of metals removed. It has been reported, for instance, that 50% demetallization can result in much more than a doubling of the useful life of a hydrotreating catalyst (see Oil & Gas Journal, April 28, 1975, pages 59-63).
Although the major reason for demetallizing a feedstock is to protect the hydrotreating and cracking catalysts, other reasons are also persuasive. For example, residual fuel oils containing excessive quantities of vanadium and nickel are virtually unsaleable because during combustion of the fuel oil in boilers these elements combine with sulfur to form corrosive compounds which rapidly destroy boiler tubing. Likewise, the coke produced from coking processes must have a limited amount of sulfur, vanadium and nickel if it is to be saleable. Thus, it is seen that demetallization of petroleum oils is necessary not only for refinery operations but also for the production of marketable products.
It has now been discovered that certain microporous catalysts comprising manganese on alumina are very effective for the demetallization of petroleum residual stocks. This result is considered surprising inasmuch as the prior art has generally focused on macroporous catalysts for this purpose, the general assumption being that catalysts having most of their total pore volume distributed in pores of diameters &lt; 200 A do not permit sufficient penetration of the large metal-containing organic molecules into the catalyst, and thus are limited in their ability to retain vanadium and other metals. Additionally, it has also been found that the demetallization process of this invention can be conducted in an inert, pressurized environment as well as in the presence of pressurized hydrogen, although better results are obtained with the use of hydrogen.